This invention relates to magneto-optic recording media which are capable of storing digital information by means of the thermal magnetic recording principle.
Several structures have been used in fabricating magneto-optic data storage media. One of the simplest structures, shown in FIG. 1, includes a substrate 12, a magneto-optic layer 11, and a first dielectric layer 10. As with the other structures discussed below, the magneto-optic layer is typically a polycrystalline thin film of MnBi, MnSb, MnCuBi, or MnTiBi; an amorphous rare earth-metal alloy such as TbFeCo, GdCo, GdFe, TbFe, GdTbFe, DyFe, or TbDyFe; or a multilayer film such as alternating thin films of Pt and Co. As shown in FIG. 1, the magneto-optic layer is sandwiched between a substrate material and a dielectric layer to provide a storage function based on the direction of magnetization in different regions of the magneto-optic layer. The digital information stored in magneto-optic layer 11 is read by directing a polarized laser beam 2 through dielectric layer 10 to magneto-optic layer 11, where it is partially reflected. An external lens (not shown) focuses laser beam 2 on magneto-optic layer 11. Depending on the magnetization direction of the segment of magneto-optic layer 11 from which laser beam 2 is reflected, the polarization direction of the reflected portion of laser beam 2 will rotate through either a positive or a negative angle, as a result of the Kerr effect. An external sensing device (not shown) reads the information stored in magneto-optic layer 11 by detecting the direction of the angular rotation of the reflected laser beam 2.
Several characteristics are significant in evaluating the performance of a magneto-optic storage medium. Among these are the signal-to-noise ratio of the reflected light beam, the ease of writing on the medium, the reliability of the medium in storing data, the susceptibility of the magneto-optic layer to corrosion, the prevention of interference from dust particles on the surface of the medium, and the reflectivity of the medium, which is important for tracking purposes.
While the structure shown in FIG. 1 is relatively easy to construct, it is vulnerable to interference from dust particles on the upper surface of dielectric layer 10, and its signal-to-noise performance is less than desirable. An improved signal-to-noise ratio is exhibited by the quadrilayer structure shown in FIG. 2, which includes a second dielectric layer 13 and a metallic reflecting layer 14 sandwiched between magneto-optic layer 11 and substrate 12. Reflecting layer 14 typically consists of a thin film of a metal such as Al, Cu, Ag or Au. In this structure, the portion of laser beam 2 which passes through magneto-optic layer 11 and second dielectric layer 13 is reflected at the surface of reflecting layer 14. Provided that the thickness of second dielectric layer 13 is set properly, the portions of laser beam 2 that are reflected at the interface of magneto-optic layer 11 and second dielectric layer 13 and the interface of second dielectric layer 13 and metal reflecting layer 14, respectively, interfere constructively so as to magnify the Kerr rotation of the reflected light, thereby improving the signal-to-noise ratio.
FIG. 3 shows an alternative structure which has the additional advantage of being less susceptible to interference from dust contamination. The structure shown in FIG. 3 is identical to that shown in FIG. 2, except that substrate 12 is positioned on the side of the quadrilayer stack upon which the laser beam is incident. Layers 10, 11, 13 and 14 function in the same manner as in the arrangement of FIG. 2. A particle of dust on the upper surface of substrate 12 is far above the focal point of laser beam 2, which is in magneto-optic layer 11. Therefore, unless the particle is extremely large, it will not affect the performance of the disk. Furthermore, if substrate 12 and first dielectric layer 10 are made of materials which have refractive indexes of approximately 1.5 and 2, respectively, the positioning of first dielectric layer 10 immediately beneath substrate 12 will help to reduce the portion of laser beam 2 (not shown) which is reflected before it reaches magneto-optic layer 11. This leads to a stronger reflected signal. Also, first dielectric layer 10 acts as a barrier to protect magneto-optic layer 11 from corrosion as a result of humidity or contamination from substrate 12. This quadrilayer structure realizes almost the same improved signal-to-noise performance as the structure of FIG. 2.
Despite the improvements inherent in the quadrilayers shown in FIGS. 2 and 3, until now structures of this type which have been designed to maximize the signal-to-noise ratio have exhibited a low degree of reflectivity, often in the range of 4 to 6%, and often an unacceptably large phase shift in the magneto-optically rotated reflected light. The servomechanisms used in disk drives for automatic focusing and tracking of the laser beam at present require a reflectivity in the range of 15 to 25%, preferably about 20%.
Phase shift in the reflected light occurs when the respective components of the reflected light parallel and perpendicular to the polarization direction of the incident beam are not in phase, resulting in elliptical polarization. Such phase shift (or ellipticity) in the reflected signal creates detection problems. If .phi. represents the phase shift of the reflected light, i.e., the phase difference between the magneto-optic reflected light component and the normal reflected light component, the performance of the information reading operation varies in proportion to cos .phi.. Therefore, a value of .phi. equal to zero is ideal, but a value of .phi. in the range of about -12.degree. to +12.degree. is considered satisfactory.
Although optical retardation plates are available to reduce or eliminate ellipticity, the utility of an individual plate is limited to a particular value of .phi.. Hence, retardation plates are not a practical solution to the problem, since the value of .phi. may vary widely between different storage media. Therefore, the multilayer structure has to be made so that the phase shift .phi. is essentially zero in order to ensure interchangeability among different media and drives.
Several studies have attempted to find an optimum quadrilayer structure in terms of the composition and thickness of each layer. See, for example, articles entitled "Design Concept of Magneto-Optical Disk", by Tamada et al., Internal Symposium on Optical Memory, Sept. 26-28, 1989, Kobe, Japan, p. 83; "Magneto-optical disk with reflecting layers", Ohta et al., SPIE, Vol. 382, p. 252, 1983; "High quality magneto-optical disk", by Takahashi et al., SPIE, Vol. 695, p. 65, 1986, all of which articles are incorporated herein by reference in their entirety.